Hilgardite is a borate mineral

Borate mineral

The borate minerals are minerals which contain a borate anion group. The borate units may be polymerised similar to the SiO4 unit of the silicate mineral class. This results in B2O5, B3O6, B2O4 anions as well as more complex structures which include hydroxide or halogen anions...

 with the chemical formula Ca₂B₅O₉Cl·H₂O. It is transparent and has vitreous luster

Lustre (mineralogy)

Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....

. It is colorless to light pink with a white streak. It is rated 5 on the Mohs Scale.
It crystallizes in the triclinic crystal system. Crystals occur as distorted tabular triangles and are hemimorphic, polytypes

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

 exist.

It was named for geologist Eugene W. Hilgard

Eugene W. Hilgard

Eugene Woldemar Hilgard was a German-American expert on pedology...

 (1833–1916). It was first described in 1937 for an occurrence in the Choctaw Salt Dome of Iberville Parish, Louisiana

Louisiana

Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...

, USA. It occurs as an uncommon accessory mineral in evaporite

Evaporite

Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...

 deposits and salt dome

Salt dome

A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....

s worldwide. In addition to the type locality

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

 it has been reported in Wayne County, Mississippi

Wayne County, Mississippi

-Demographics:As of the census of 2000, there were 21,216 people, 7,857 households, and 5,853 families residing in the county. The population density was 26 people per square mile . There were 9,049 housing units at an average density of 11 per square mile...

 and in the Louann Salt

Louann Salt

The Louann Salt is a widespread evaporite formation that formed in the Gulf of Mexico during the Callovian in the mid Jurassic. The Louann formed in a rift as the South American and North American Plates separated, from an embayment of the Pacific Ocean...

 Formation, Clarke County, Alabama

Clarke County, Alabama

-2010:Whereas according to the 2010 U.S. Census Bureau:*54.5% White*43.9% Black*0.4% Native American*0.3% Asian*0.0% Native Hawaiian or Pacific Islander*0.7% Two or more races*1.0% Hispanic or Latino -2000:...

 in the United States and at the Penobsquis and Salt Springs evaporites, near Sussex, New Brunswick

Sussex, New Brunswick

Sussex is a Canadian town in Kings County, New Brunswick.Sussex straddles the Kennebecasis River, 70 km northeast of Saint John, and is a major dairy products producer in the province...

, Canada. In Europe it is reported from the Konigshall-Hindenburg potash

Potash

Potash is the common name for various mined and manufactured salts that contain potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains, and this was the major historical source for it before the industrial era...

 mine near Göttingen

Göttingen

Göttingen is a university town in Lower Saxony, Germany. It is the capital of the district of Göttingen. The Leine river runs through the town. In 2006 the population was 129,686.-General information:...

, Lower Saxony

Lower Saxony

Lower Saxony is a German state situated in north-western Germany and is second in area and fourth in population among the sixteen states of Germany...

, Germany and in the Boulby potash mine, Whitby

Whitby

Whitby is a seaside town, port and civil parish in the Scarborough borough of North Yorkshire, England. Situated on the east coast of Yorkshire at the mouth of the River Esk, Whitby has a combined maritime, mineral and tourist heritage, and is home to the ruins of Whitby Abbey where Caedmon, the...

, Yorkshire

Yorkshire

Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...

, England. In Asia it is reported from the Chelkar salt dome, Uralsk district, Kazakhstan

Kazakhstan

Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

; the Ilga Basin, eastern Siberia

Siberia

Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

, Russia and the Sedom Formation, Mount Sedom, Dead Sea

Dead Sea

The Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...

, Israel

Israel

The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

.